1. Field of the Invention
The invention relates to electronic circuitry for amplifying signals. More specifically, the invention relates to a monolithic circuit which may be fabricated on a substrate and which operates as an amplifier.
2. Description of the Prior Art
Electronic circuits for amplifying signals have long been known. Specifically, circuits are known for amplifying electrical signals at microwave frequencies. A typical small signal microwave amplifier, for example, includes an active device such as a field effect transistor (FET), with input and output impedance matching circuits. The fundamental design constraints on such an amplifier are that it provide a specified gain over a specified frequency bandwidth.
The active device, such as a FET, provides gain, and it is well known that maximum gain is obtained when the input and output impedances of the device are conjugately matched. On the other hand, the input and output impedances are complex, having both resistive and reactive components, so that the input and output impedances vary with frequency. Therefore, the input and output impedances cannot be matched at all frequencies, and some impedance mismatching between the input and output of the active device is necessary to obtain a specified bandwidth. As a result, the gain of the active device will be less than its maximum, which is not especially troublesome due to the high maximum available gain of FET devices.
The mismatched impedances at the input and output of an active device can be troublesome, however, if several such devices are connected together as cascaded amplifying stages. The difference between the output impedance of one stage and the input impedance of the next stage will cause signal reflections, resulting in signal attenuation and high gain ripple. This problem will also occur if the output impedance of the signal source does not match the input impedance of the first stage. To minimize the problems caused by signal reflection, isolators may be connected between amplifier stages, or hybrid amplifiers having balanced stages may be used. Isolators and hybrid amplifiers, however, are not amenable to monolithic fabrication, precluding a monolithic cascaded amplifier on a substrate.
A similar problem results at the output of a cascaded amplifier, because the load must be transformed to the high output resistance, roughly 200-600 ohms, of an FET device. At microwave frequencies, the elements which would be necessary to transform the load cannot be realized, so that a compromise must again be made between load matching and gain, resulting in lower gain.
It would be advantageous to provide an electronic amplifier circuit which would permit cascading directly, rather than with isolators or by balancing stages. It would also be advantageous to have an amplifier which could be fabricated in monolithic form on a substrate, permitting batch fabrication, which would result in lower cost, higher reliability, and reductions in size and weight as great as several orders of magnitude. It would also be advantageous to provide such an amplifier with a simple configuration requiring no critical tuning elements for broad band gain. It would also be advantageous to provide such an amplifier with a very small impedance mismatch across a wide bandwidth.